The present invention relates to a method of producing an optically molded product by injection compression molding. More specifically, it relates to a method of producing an optically molded product which is required to have excellent surface accuracy and optical properties, such as a spectacle lens, from a transparent thermoplastic resin such as a polycarbonate resin by injection compression molding.
Demand for plastic lenses has recently been growing. Plastic lenses are roughly divided into acrylic resin lenses produced by casting-thermosetting molding and thermoplastic resin lenses such as polycarbonate resin and polyacrylic resin lenses produced by injection molding.
Particularly, polycarbonate resin lenses have been attracting much attention as spectacle lenses which can be produced by injection molding and have a high refractive index, light weight, excellent ultraviolet light absorptive power and safety with impact resistance and have been used recently in large quantities.
There are proposed a large number of methods of producing spectacle lenses by injection molding. Known methods include one in which a semi-finish lens is produced by injection molding and shaped into an optical form of interest by cutting and polishing and one in which a lens having an optical form is obtained by a single injection molding process as a finish lens. Particularly, the latter case involves a basic problem caused by the form of a lens that when a concave lens is to be produced by injection molding, a molten resin from a gate flows fast in a portion corresponding to the peripheral portion of the lens because it is thick and slow in a portion corresponding to the center portion of the lens because it is thin in a cavity.
As a result, resin flows meet each other in the peripheral portion in the end, forming a weld line. This is marked as the focusing distance of the concave lens becomes shorter, thereby deteriorating the optical properties of the lens and greatly impairing the appearance of the lens. Depending on the outer diameter of a lens, the formation of the weld line which is a quality problem rarely occurs in lenses having a focusing distance of about xe2x88x921 m or less. However, the formation of the weld line easily occurs in concave lenses having a focusing distance of xe2x88x921 m or more, particularly concave lenses having a focusing distance of xe2x88x920.5 m or more.
There are proposed various methods of solving the above problem caused by the injection molding of spectacle lenses. Out of the methods, typical methods for eliminating the weld line include one in which an overflow pocket is formed in a peripheral portion to limit a resin flow in the peripheral portion relatively as disclosed by JP-B 61-19409 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d), one in which a side core is formed in a cavity as disclosed by JP-A 62-83121 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), and one in which very small irregularities are formed on the most peripheral portion of a lens to limit a resin flow in the peripheral portion as disclosed by JP-A 1-90716.
However, when the overflow pocket or the side core is formed in the cavity, the structure of a metal mold becomes complex, the peripheral portion of a powerful concave lens (may be referred to as xe2x80x9cminus lensxe2x80x9d) becomes very thick, and a weld line cannot be eliminated substantially in these methods.
Since a flow of a molten resin in the cavity is disturbed in the methods in which the side core or irregularities are formed in the cavity, a molding defect such as a flow mark or a cold flow is easily induced and the releasability of a lens molded product is impaired. A cylindrical surface or toric surface on the most peripheral portion of which changes in thickness cannot be formed with these methods.
Further, as the production of a spectacle lens by injection molding is generally carried out by a method of molding a great quantity and multiple types of spectacle lenses at the same time, it is very complicated to attach and detach accessories to and from the cavity, thereby greatly limiting the type of spectacle lenses which can be molded.
Although a multi-stage compression method makes it possible to obtain a lens having excellent surface accuracy, the formation of a weld line cannot be prevented basically because the amount of a resin corresponding to the amount of shrinkage is filled.
Thus, there are proposed a large number of methods of preventing the formation of a weld line, such as one in which the temperature of a metal mold is controlled in a complicated manner and one making use of ultrasonic waves. However, these methods involve such problems as the complex structure of a metal mold and the limited design of a lens. A method of molding an optical lens having substantially no weld line is not yet to be established.
When a finish lens is to be produced by injection molding, a distortion or a reduction in surface accuracy is easily caused by the shrinkage of a resin due to solidification by cooling. This phenomenon is more marked as the shrinkage difference of a molded product having a large thickness difference becomes larger. To eliminate a shrinkage difference caused by solidification by cooling, there is a typical one as disclosed by JP-B 6-71755 in which a multi-stage compression method is carried on while the amount of a resin equivalent to the amount of shrinkage is filled in advance. However, this method has such problems as insufficient surface accuracy and large differences among molded products. The expression xe2x80x9csurface accuracyxe2x80x9d as used herein means whether surface curvature, flatness and the like fall within designed standard ranges.
These known injection compression molding methods have the following two defects. One of the defects is that a sufficient injection resin pressure is not applied at the time of the completion of injection when the surface layer of a molded product which exerts a great influence upon the surface accuracy and optical distortion of the molded product is formed because a resin is filled into a cavity and a sufficient injection resin pressure is not applied at the time of injection. Therefore, a distortion or poor surface accuracy results. The expression xe2x80x9coptical distortionxe2x80x9d as used herein means an optical distortion which can be easily observed with the naked eye, an optical distortion which can be observed from an image of a fluorescent light reflected on the surface of a lens, an optical distortion which can be seen as a thin ring form by a polarizing plate, or the like. These defect phenomena can be easily discovered the most by observation with a polarizing plate. These are fatal defects when the molded product is used as a lens. These defect phenomena easily occur in the central portion of a plus lens and the peripheral portion of a minus lens. The cause of these phenomena is mainly that a sufficient resin pressure cannot be applied to the inside of the cavity at the time of the completion of injection when the surface layer of a lens is formed.
The second defect is that the filled state of a resin in the injection step of an injection molding machine includes differences in the injection step or differences in the metering step and the state of a resin in the cavity before compression often greatly differs for each molding shot. Therefore, there often produced differences above a tolerable range from the viewpoint of the quality control of an optically molded product. The injection compression molding of the prior art is greatly affected by the differences.
It is therefore a first object of the present invention to provide a method of molding an optically molded product which has no weld line or a very small weld line if it is formed.
It is a second object of the present invention to provide a method of molding an optically molded product having excellent surface accuracy.
It is a third object of the present invention to provide a method of molding an optically molded product, which facilitates molding multiple kinds and a large quantity of molded products at the same time without using a cavity having a complex shape.
It is another object of the present invention to provide a method of molding an optically molded product, which has small differences of quality molded products and which can mold a high-quality molded product stably and industrially advantageously.
It is a further object of the present invention to provide a method of molding an optically molded product economically advantageously.
According to studies conducted by the inventors of the present invention, the above objects of the present invention are attained by a method of producing an optically molded product of a thermoplastic resin by injection compression molding, the method comprising the steps of:
(1) expanding the volume of a cavity more than the volume of the optically molded product of interest;
(2) injecting a molten thermoplastic resin into the cavity through an injection cylinder;
(3) compressing the expanded cavity to a prescribed thickness of the molded product;
(4) returning an excess of the thermoplastic resin produced by the compression into the injection cylinder;
(5) keeping the molten thermoplastic resin in the cavity until the molded product of interest is formed; and
(6) taking out the obtained molded product from the cavity.
This molding method will be referred to as xe2x80x9cfirst molding methodxe2x80x9d hereinafter.
Further, according to studies conducted by the inventors of the present invention, the above objects of the present invention are attained by a method of producing an optically molded product of a thermoplastic resin by injection compression molding, the method comprising the steps:
(1) expanding the volume of a cavity more than the volume of the optically molded product of interest;
(2) injecting a molten thermoplastic resin into the cavity through an injection cylinder;
(3) compressing the expanded cavity to a prescribed thickness of the molded product or a thickness 200 xcexcm smaller than the thickness;
(4) adjusting or changing resin pressure in the injection cylinder and compression pressure in the cavity in limits that the change width does not exceed 100 xcexcm from the prescribed thickness of the molded product to achieve the prescribed thickness of the molded product in the end;
(5) keeping the molten thermoplastic resin in the cavity until the molded product of interest is formed; and
(6) taking out the obtained molded product from the cavity.
This molding method will be referred to as xe2x80x9csecond molding methodxe2x80x9d hereinafter.
The expression xe2x80x9coptically molded productxe2x80x9d as used herein denotes an optically molded product obtained by forming an image of an object using light refraction and reflection and diverging or converging a pencil of rays, or an optically molded product obtained making use of an interference phenomenon or divergence caused by the phase difference of laser beams. Illustrative examples of the optically molded product include plastic spectacle lenses and projector lenses. The present invention is particularly advantageous for the molding of a polycarbonate resin spectacle lens.
Since a resin is shut off from a runner by such means as a gate seal and then a cavity is compressed in the injection compression molding method of the prior art, an excess of the resin is stored in an overflow pocket or the like. Since a small amount of the filled resin for making up for the amount of shrinkage is existent, a complex metal mold structure is required for inhibiting the forming of weld line to provide a side core or irregularities, control the temperature of a metal mold or make an ultrasonic treatment.
The inventors of the present invention ventured to study means of returning a molten resin into the injection cylinder, which has been considered inappropriate in the prior art injection molding technology, and could establish an injection compression molding method which can handle a small excess or large excess of the resin with very simple means and rarely forms a weld line for a lens having any shape.